This invention relates generally to lasers, and, more particularly to a single axis resonator for a laser having an annular gain region.
The operation of a laser is based upon the fact that the atomic systems represented by the atoms of the laser medium can exist in any of a series of discrete energy levels or states, the systems absorbing energy in the optical frequency range in going to a higher state and omitting it when going to a lower state.
The laser medium may be a solid, liquid or gas. In a case of a solid wherein a ruby is used as a laser material, three energy levels are utilized. The atomic systems are raised from the lower or ground level to the higher of the three levels by irradiation from, for example, a strong light source which need not be coherent but should preferably have a high concentration of energy in the absorbing wavelengths. A radiationless transition then occurs from the highest state to an intermediate or metastable state. This is followed by a transition with photon emission from the intermediate state back to the ground state. It is the last transition that is of interest since this transition is the source of the coherent light or electromagnetic energy produced by the laser.
The operation of raising the energy level of the laser material to produce the desired photon emission is referred to in the art as "pumping" and when more atoms reach an excited metastable state than remain in a lower energy level a "population inversion" is said to exist. The active medium in the laser is made optically resonant by placing reflectors or other optical devices, hereinafter referred to as the resonator of the laser, at the ends thereof, forming the resonant chamber therebetween. The resultant laser beam escapes from the resonant chamber.
Generally, gas systems are preferred for high average power lasers. Gas lasers are conventionally arranged to have gas flow through the resonant cavity or gain region.
Gas lasers are classified in accordance with the process by which the gas laser medium achieves the population inversion. Three conventional varieties of gas lasers are chemical lasers, electric discharge lasers and gas dynamic lasers. Chemical lasers achieve the population inversion by direct generation of higher energy vibrational states in the products of a chemical reaction. Electric discharge lasers achieve the population inversion by "pumping" the higher energy vibrational states in the media through the action of an electric current as in the manner set forth above with respect to the ruby laser. Gas dynamic lasers achieve the population inversion by reducing the population level of the lower energy vibrational state of a hot gas in thermal equilibrium through the rapid cooling caused by a supersonic aerodynamic expansion.
Since the possible application of high power lasers are unlimited in the fields of communication, manufacturing, construction, medicine, space exploration and defense, research in this area is ever expanding. In fact, it has now been shown that high energy lasers may be made more efficient by constructing these lasers of a cylindrical configuration. As a result, they provide a gain region which is annular in shape. Unfortunately, extracting optical power from an annular gain region has proven to be a difficult problem to solve.